The overarching aim of this research is to promote children's positive, adaptive pathways of socio-emotional development and to prevent maladaptive pathways from infancy to adolescence. Our basic research seeks to explain why some children embark on positive developmental paths toward a mature conscience that encompasses appropriate moral emotions, prosocial, internalized, rule-abiding conduct, and robust socio- emotional competence when functioning in the family and broader ecologies, and why other children enter maladaptive paths toward callousness, disregard for rules and others' feelings, disruptive, antisocial, high-risk behavior, poor competence, and impoverished socio-emotional growth. Although the role of parental socialization is broadly acknowledged, specific mechanisms of its impact for individual children are not yet understood. In this research, early parent-child Mutually Responsive Orientation (MRO) is proposed as a powerful factor promoting positive long-term socio-emotional outcomes. In the context of MRO, socialization becomes a shared, reciprocal parent-child enterprise, and the child becomes an active, receptive, and willing participant, eager to embrace parental goals and values. Consequently, the parent can relinquish the use of aversive, heavy-handed control. This research further elucidates how socialization processes are strongly interwoven with, and their outcomes dependent on the individual child's biological individuality. Our ongoing longitudinal Family Study of 102 community mothers, fathers and children has supported this model using massive data collected when children were 7, 15, 25, 38, 52, 67, 80, and 96 months. New assessments are proposed at 10-11, 12-13, and 14-15 years, to examine implications of early MRO during the critical transition to adolescence. Our translational research, Play Study, an ongoing, theory-informed, randomized parenting intervention with 186 low-income, diverse mother-toddler dyads, fosters early mother-child MRO, and thus promotes socialization paths leading to positive child outcomes. Data are collected before, throughout, immediately after and 6 months after the intervention. A proposed new assessment at age 7-8 will test its long-term impact following the children's key transition to school. This work is significant because both studies are synergistic and parallel regarding aims, constructs, and measures, but complementary regarding designs and populations, and employ massive, multi-method, multi-trait, multi-assessment, multi-informant measures of children's genotypes, psychophysiology, temperament, behavior and emotions, conscience, comprehensively assessed competencies and functioning, parent-child relationships, and parents' adjustment and family ecology. Interdisciplinary team includes experts in socio-emotional development, adolescence, developmental psychopathology, maternal adjustment, intervention science, molecular genetics, and statistics. An innovative, unified theoretical framework integrates both studies. Analyses elucidate changes over time, divergence in developmental trajectories, processes and causal mechanisms linking constructs (mediation), and multiple causal pathways (moderation), using structural equations modeling (SEM).